Experimental dynamic out-of-plane response of masonry buildings: mechanism activation, progression, and collapse

Many uncertainties are related to the seismic out-of-plane (OOP) response of unreinforced masonry (URM) structures. Challenges include the prediction of the triggering acceleration, velocity, and their role on the final collapse mechanism. To date, shake-table tests have investigated limited boundary condition scenarios, and the common unrestrained wall-to-wall and wall-to-diaphragm connections lack investigation. These conditions lead to hybrid two-way bending, a response featuring simultaneous rocking and horizontal bending, so far observed only in a static environment. This paper discusses an extensive shake-table campaign comprising 211 tests to observe the OOP response of URM specimens with no top horizontal connection subjected to structural and input variations. A novel methodology combining dynamic testing and Digital Image Correlation (DIC) techniques is proposed. Mechanism interaction is addressed in three instances, namely activation, progression, and near-collapse. Results demonstrate the existence of hybrid two-way bending, characterised by sustained horizontal one-way bending cycles, and short, sudden rocking events. Five distinct collapse mechanisms are observed, thereby strengthening the literature on the issue. The campaign has generated a robust experimental dataset, which supports validation of analytical and numerical models, as well as further probabilistic analyses (i.e., fragility analysis).